1. Field of the Invention
The present invention relates to an electrophotographic image forming apparatus that forms an image with an electrophotography system and an image forming method.
2. Description of the Related Art
Some of electrophotographic image forming apparatuses, such as copying machines and printers, use a laser scan method for forming an image by deflecting a laser beam emitted from a laser source for scanning. An optical scanning system using this laser scan method employs a configuration in which a beam emitted from a laser source is deflected by a polygon mirror, and is converged onto a photoconductive drum for scanning through a collimator lens and an f-theta lens, in general.
On the other hand, an image forming apparatus using an LED method for forming an image by LEDs (an LED array) aligned in the longitudinal direction of the photoconductive drum is known. The LED method employs a configuration in which an LED head, which integrates the LED array and a rod lens array for converging lights emitted from the LED array onto the photoconductive drum, is arranged over the photoconductive drum.
In either method, if the positional relationship between the light source and the lens, or the positional relationship between the lens and the photoconductive drum deviates from a predetermined condition, the diameter of the light spot formed on the photoconductive drum increases. For example, if a frame etc. of the image forming apparatus deforms due to heat generated by driving the image forming apparatus, the positional relationship among the light source, the lens, and the photoconductive drum deviates, which changes the light path length between the light source and the photoconductive drum. Thereby, the diameter of the light spot (it is described as “spot diameter”) on the photoconductive drum varies.
When the spot diameter increases, light spots overlap over adjacent dots or lines (it is called as “interference of light spots”), which causes problems, such as a variation in a density of a halftone image. Since the distance between the lens and the photoconductive drum in the LED method is shorter than that in the laser scan method, the ratio of the increase in the spot diameter to the change of the distance between the lens and the photoconductive drum is particularly higher in the LED method.
Accordingly, there is a known method of using a mechanical adjustment mechanism that adjusts a distance between an LED head and a photoconductive drum for controlling increase in a spot diameter. On the other hand, Japanese Laid-Open Patent Publication (Kokai) No. 2002-55498 (JP 2002-55498A) discloses a method for detecting state of a light spot based on an image sample to adjust a density of a halftone image according to the state of light spot. This publication describes that the variation in the density of a halftone image due to the increasing spot diameter can be corrected while avoiding cost increase due to an addition of an adjustment mechanism because the method disclosed in this publication does not need to use a mechanical adjustment mechanism.
However, since the method disclosed in JP 2002-55498A adjusts the density of a halftone image by an image process according to the spot diameter, the optimal density adjustment value varies depending on image resolution of image data. Accordingly, there is a problem that the density adjustment remainder remains. Here, a relation among a variation of spot diameter, image resolution, and a density will be described.
In an image like a halftone image in which pixels without data (no-lighting pixels) and pixels with data (lighting pixels) are intermingled, a density tends to vary because exposure area of one pixel varies depending on variation in the spot diameter. Particularly, when the image resolution is high (when a screen ruling is large, for example), the density fluctuation amount due to the variation of spot diameter tends to become large.
FIG. 8A through FIG. 8D are views schematically showing increases of spot diameter due to defocus. FIG. 8A shows dot shapes under the condition where two lighting pixels (light emission points A and B) are positioned with a fixed short distance interval (distance d2) therebetween, and light beams optimally focus to a photoconductive drum (a just focus state). FIG. 8B shows dot shapes of the lighting pixels A and B under the same condition as FIG. 8A when spot diameters increase due to defocus. Since the increases of spot diameters cause an overlap of the dots when the distance between the lighting pixels is short as shown in FIG. 8A and FIG. 8B, an image density under the defocus state shown in FIG. 8B varies significantly as compared with the just focus state shown in FIG. 8A.
FIG. 8C shows dot shapes under the condition where two lighting pixels (light emission points A and B) are positioned with a fixed long distance interval (distance d3) therebetween, and light beams optimally focus to the photoconductive drum (the just focus state). FIG. 8D shows dot shapes of the lighting pixels A and B under the same condition as FIG. 8C when spot diameters increase due to defocus. Here, it is assumed that the distance d3 is longer enough than the distance d2. Since the increases of spot diameters do not cause an overlap of the dots when the distance between the lighting pixels is long as shown in FIG. 8C and FIG. 8D, a density fluctuation amount can be reduced as compared with the case shown in FIG. 8A and FIG. 8B.
For such a reason, the density fluctuation amount generated in response to the variation of spot diameter varies depending on the distance interval between lighting pixels. Particularly, since lighting pixels and no-lighting pixels are positioned with short distance intervals in a halftone image with large screen ruling, the density tends to vary due to the increase in the spot diameters.
Although the apparatus disclosed in JP 2002-55498A switches image processing method based on a determination of whether inputted image data is character/line data or not, there is a problem that the density fluctuation cannot be corrected accurately because the same process is applied to image data regardless of difference in screen ruling. If a system switches a processing method by determining screen ruling and a type of image data, the system needs to provide a circuit for determining the type of image data, and correction tables for the respective types of image data. This complicates and enlarges the circuit, and increases a cost.